Alkane isomerization processes are widely used by refiners to convert normal C.sub.4 and C.sub.5 alkanes and normal and mono-methyl-branched C.sub.6 alkanes into more valuable branched alkanes. The multi-methyl-branched C.sub.6 alkanes have a higher octane number and are used as gasoline blending components to boost the octane number of the gasoline. The mono-methyl-branched C.sub.4 and C.sub.5 alkanes may also be used as intermediates, after dehydrogenation, for such oxygenate products as methyl tertiary butyl ether, ethyl tertiary butyl ether, and tertiary amyl methyl ether.
Typically, commercial isomerization processes have had at least a two-stage design; the first stage is a fixed bed reactor, and the second stage is a separation unit. See, for example, U.S. Pat. Nos. 5,146,037 and 5,245,102. The isomerization that takes place in the fixed bed reactor is limited by thermodynamic equilibrium, which results in the reactor effluent containing a substantial amount of unconverted alkanes. The separation unit, which is usually either an adsorption or a fractionation unit, is used to separate the unconverted alkanes from the isomerized product alkanes. The unconverted alkanes are generally recycled to the fixed bed reactor. With this type of design, the recycle stream is usually substantial, and methods of increasing the yield of highly branched alkanes are in demand.
Normal and mono-methyl-branched alkanes containing 7 or more carbon atoms have been converted into benzene and other valuable aromatic hydrocarbons for gasoline blending by catalytic reforming. However, due to environmental concerns, the demand for aromatics in the future may diminish. An alternate refining process for the normal and mono-methyl-branched C.sub.7 and C.sub.8 alkanes that yields a high octane number product is the present invention of alkane isomerization by reactive chromatography. Prior to the current invention, C.sub.7 and C.sub.8 alkanes were not isomerized due to the extensive cracking of the desired highly branched alkane products. However, with the rapid removal of the highly branched products in the current invention, the amount of cracking is reduced and the product dimethyl- and trimethyl- C.sub.7 and C.sub.8 alkanes are formed. The isomerized C.sub.7 and C.sub.8 products may be used as octane number boosters in gasoline blending instead of benzene and other aromatics.
The present invention makes use of both simulated moving bed technology and reactive chromatography to perform isomerization of hydrocarbons containing from about 4 to about 8 carbon atoms. Reactive chromatography allows for concurrent isomerization and separation of the unconsumed reactants from the products, thereby extending product yields beyond thermodynamic equilibrium limitations. Others have attempted concurrent alkane isomerization and separation, but only using fixed bed systems. For example, Badger, C. M. A.; Harris, J. A.; Scott, K. F.; Walker, M. J.; Phillips, C. S. G. J. Chromatogr. 1976, 126, 11-18, disclosed placing a catalyst in a gas chromatography column and having a heater move along the length of the column to catalyze isomerization and effect separation.
Also, U.S. Pat. No. 4,783,574 disclosed a fixed bed reactor containing two sub-beds of adsorbent at opposite ends of the reactor and one sub-bed of catalyst in the center of the reactor. The feed was introduced near the catalyst sub-bed, and a desorbent was introduced at one end of the reactor. The isomerization was catalyzed and unconsumed reactants were adsorbed on the adsorbent sub-bed downstream of the catalyst sub-bed in the direction of the desorbent flow. Then the desorbent flow was reversed by introducing the desorbent from the opposite end of the reactor to desorb the unconsumed reactants and carry them back to the catalyst sub-bed. The present invention is significantly distinct from the art. The present invention is operated in a simulated moving bed mode, and incorporates a homogeneous mixture of catalyst and adsorbent in every sub-bed.